This invention relates generally to clutches and more particularly to spring clutches that include a coil spring that is wound tightly around a shaft to transfer toque to the shaft.
Devices such as automotive lift gates, side doors, deck lids and latches are often power operated by a drive mechanism that has an electric motor as the prime mover. With such devices it is desirable to provide for manual operation of the device in the event of power failure. Ideally, manual operation of the device does not take any more effort than an installation without the drive mechanism.
Some power operated devices, such as power operated lift gates and side doors include an electromagnetic clutch in the drive mechanism. The magnetic clutch allows the drive mechanism to operate the power operated device in either direction, for example to raise or lower the lift gate, or to open or close the side door. However, the electromagnetic clutch also automatically disengages the electric motor from the power operated device at the end of the power operation. Thus in the event of power failure the manual operating effort is reduced substantially by eliminating the requirement for back driving the electric motor. While such drive mechanisms are successful in reducing the manual operating effort, drive mechanisms incorporating electromagnetic clutches are complex and costly due in large part to the electrical operation of the electromagnetic clutch.
This invention provides a spring clutch that can have all the capabilities of an electromagnetic clutch with respect to driving a power operated device in either direction and automatically disengaging at the end of the power operation so that the device can be operated manually with a reduced effort.
In one aspect, the invention provides a spring clutch that is actuated by a magnetic brake so that an input shaft drives an output shaft in at least one direction and the spring clutch automatically disengages at the end of the driving operation so that the output shaft rotates freely in either direction.
In another aspect, the invention provides a bidirectional spring clutch actuated by a magnetic brake. Here, an input shaft that drives an output shaft in either direction and the spring clutch automatically disengages at the end of the driving operation so that the output shaft rotates freely in either direction.
In either aspect, the spring clutch includes a magnetic brake that operates on the Lorenz principle. As a conductive, non-magnetic metal, such as copper, is moved past a magnet, an opposing or drag force is generated by eddy currents set up in the conductive, non-magnetic metal. The drag force is proportional to the speed of the movement until magnetic saturation occurs at which time, the drag force becomes constant. This drag force is produced in response rotation of an input shaft and when sufficient engages a spring clutch that couples the input shaft to an output shaft for torque transfer. When speed of the input shaft drops below a predetermined value, the drag force is overcome by the spring force and the spring clutch disengages automatically. Thus when the input shaft stops, the output shaft can be rotated freely in either direction. Since the drag force is created without any contact as in the case of friction produced drag force, there is not any wear and the spring clutch will operate with consistent long term performance.
The spring clutch contains a coil spring that is attached to the input shaft at one end and to a rotor of the magnetic brake at the other end. The coil spring is wound around the output shaft and wound down tight onto the output shaft by the magnetic brake resulting is torque transfer in one direction in the case of a one-way spring clutch. The bi-directional spring clutch operates by using two springs wound in opposite directions. The two springs can be connected by a collar or wound from a single piece of wire.